The present invention relates to laser material processing, and more specifically to a method and a device for protecting a laser unit from dust or debris produced during laser processing of a target or substrate in a processing area.
The present invention is especially useful in connection with an apparatus for laser engraving or marking. Therefore, the technical background of the invention, and objects and embodiments thereof, will be described with reference to such laser engraving apparatus. However, the invention may also be applicable in connection with equipment for any other type of laser material processing, such as other laser surface treatment, laser cutting and laser welding.
Generally, for example as disclosed in DE-A1-43 38 774 or DE-A1-44 05 203, a laser engraving apparatus comprises a laser unit and a target guiding device which is disposed below the laser unit and adapted to guide a target past the laser unit. The laser unit includes a laser head being adapted to generate laser radiation, and a control head being adapted to direct and focus the laser radiation on the target to be provided with laser engraved markings. When the target is being laser processed, material is removed from the target in the form of small particles. These particles, or debris, collectively produce dust that either adheres to the surface of the target or, by the impact of the laser radiation, flies away from the target. In such an apparatus, it is important to prevent dust from entering the laser unit, since the presence of dust potentially could lead to deterioration or destruction of any optical components included therein.
In a laser engraving apparatus disclosed in EP-A1-0 085 484, a not further described air-curtain arrangement is used in connection with a depending bellow intermediate a laser unit and an underlying engraving area to prevent ingress of dust into the laser unit. Such an arrangement might be sufficient to prevent dust contained in the surrounding air from entering the laser unit. However, dust or debris which is produced at the engraving area, as a result of material being removed from the target, might still enter the laser unit. This problem is enhanced in high-precision engraving, when the distance from the laser unit to the engraving area has to be reduced, thereby bringing the source of dust generation closer to the sensitive laser unit.
Evidently, the situation would be even worse if the laser unit were to be arranged beneath the engraving area, since gravity would promote an accumulation of dust on the laser unit. When two opposite sides of a target are to be provided with engraved markings, especially when the markings on the two sides must be precisely located with respect to one another, it is advantageous, or even necessary, to engrave the target from both sides. If the target has to be turned upside-down between the engraving operations, the positional relationship between the opposite sides is easily lost. This problem is accentuated when a continuous web of material is being engraved, since the turning operation will be complicated to achieve and requires a great deal of space. Such a turning operation is often inconsistent with high production speeds. Also, in using two consecutive engraving stations to engrave one respective side of the web, and an intermediate turning station for turning the web, the distance between the engraving stations must be so large that it is difficult to maintain a positional relationship between the markings on the opposite sides of the web.
Further, if the target to be provided with laser engraved markings is small, light-weight and/or made of flexible material, a conventional air-curtain arrangement might undesirably alter the position or shape of the target during the engraving operation.
An object of the present invention is to eliminate, or at least alleviate, the drawbacks mentioned above and to provide an improved method and device for protecting a laser unit from dust during processing of a target or substrate in a processing area. More specifically, the invention aims at essentially eliminating ingress of dust into the laser unit while still allowing for high-precision laser processing of all kinds of targets.
It is also an object of the invention to provide for minimum influence on the position and shape of the target during processing thereof in the processing area.
A further object of the invention is to provide for laser processing from either side, or both sides, of a target.
These and other objects, which will appear from the following description, have now been achieved by a method and a device as defined in the appended independent claims. Preferred embodiments of the invention are set forth in the dependent claims.
By establishing an ambient pressure at the processing area, forces acting on the target can be minimized. Thus, with respect to the dust protection measures, any kind of feasible target can be processed, even targets that are small, light-weight and/or flexible. By establishing a longitudinal gas flow directed towards the processing area in a first region intermediate the processing area and the laser unit, dust is effectively prevented from moving towards the laser unit. Further, the method and device of the invention allows for laser processing from beneath a target, since the longitudinal gas flow in the first region can be adjusted to counteract the gravitational driving force of the dust produced on the target during processing.
In one embodiment, gas is simultaneously removed adjacent to the processing area and fed to the first region intermediate the processing area and the laser unit, thereby establishing the ambient pressure at the processing area and the gas flow directed towards the processing area. Preferably, the longitudinal gas flow is accelerated towards the processing area, to further increase the ability of the gas flow to counteract the gravitational driving force of the dust.
In another embodiment, a housing is arranged intermediate the laser unit and the processing area. A channel extends within the housing from a radiation inlet opening which is facing the laser unit, and a radiation outlet opening which is facing, and generally is located close to, the processing area. Gas is simultaneously fed to and removed from the channel at separated locations therein. By balancing the feeding and removal rates of gas in the housing, it is assured that the pressure at the radiation outlet opening, and in practice also at the processing area, is maintained at essentially ambient pressure. In a simple and efficient arrangement, the balancing of the feeding and removal rates of gas is effected by controlling the gas removal rate such that essentially ambient pressure is established at the processing area.
It is preferred that the removal of gas is effected adjacent to the radiation outlet opening, since such removal will facilitate the control of the pressure at the radiation outlet opening. Furthermore, generated dust can be removed from the region around the radiation outlet opening. Thus, the target can be cleaned from dust while being processed.
Preferably, the longitudinal gas flow is established over essentially a full cross-section of the channel in the first region. In one embodiment, this is achieved by feeding gas into the first region in at least one pair of opposite inlet gas flows, which cooperate to form the longitudinal gas flow directed towards the processing area. Preferably, the opposite inlet gas flows together sweep a full cross-section of the channel in the first region. On entering the channel, the opposite inlet gas flows are preferably directed essentially perpendicular to the longitudinal center line of the channel, so that they meet and together form the longitudinal gas flow over the whole cross-section of the channel.
In another embodiment, a peripheral gas flow is separately established along a channel periphery in the first region. Preferably, this is achieved by directing one or more gas jets from the channel periphery onto the processing area. Such gas jets will counteract the formation of any wake regions close to the channel walls, particularly in any corners thereof. Additionally, such gas jets can assist in moving the generated dust from the target to the radiation outlet opening where it is efficiently removed.
Preferably, gas is removed in at least two diametrically opposite directions adjacent to the radiation outlet opening. This arrangement allows for sufficient control of the pressure at the processing area, as well as efficient removal of dust from the processing area.
In a further embodiment, a pressure barrier is established in a second region in the channel, preferably intermediate the first region and the laser unit. The pressure barrier, i.e. a region of higher pressure as seen from the processing area, will assist in preventing dust from reaching the laser unit.
In an additional embodiment, a lateral gas flow is established in a third region in the channel, preferably intermediate the first region and the laser unit. The lateral gas flow is established between opposite openings in a side wall portion of the channel, preferably in the form of a sheet of gas that sweeps a full cross-section of the channel in the lateral direction. Any dust passing the first region, and the second region if present, has been significantly decelerated and will be effectively removed by such a lateral gas flow. Preferably, the lateral gas flow is established by balancing the associated gas feeding and removal rates, so that the lateral gas flow is prevented from interacting with, and potentially disturbing, the flow field in the first region.
In one embodiment, the channel has a non-circular cross-section, at least in the first region. This configuration will prevent any formation a swirling gas motion in the first region. Such a swirling gas motion, for example around the longitudinal axis of the channel, would exhibit a central low-pressure zone, in which dust could move towards the laser unit. Preferably, the cross-section is polygonal. To counteract any formation of wake regions close to the channel walls, particularly in the corners of the polygon, it is preferred that jet-generating openings are formed in a side wall portion of the channel, preferably at the corners. Each jet-generating opening has a shape and location to provide for a gas flow along a periphery portion of the channel towards the processing area.